In the normal operation of an automotive vehicle, a fully charged six cell battery having between 2.05 and 2.1 volts per cell (hereinafter referred to as a "start" battery) is used to start the engine and to operate accessory loads when the engine is not running. The conventional start battery is well suited to provide large start currents to the start motor on the order of 150 to 250 amperes.
The start battery is provided with thin plates between its individual cells that provides for a rapid, large current, shallow discharge during vehicle start. Unfortunately, the start battery cannot be deeply discharged in a repetitive manner without damaging the thin cell separator plates.
It has long been the practice to provide an alternator driven by the engine that can be used to recharge the start battery after vehicle start and provide current to both the vehicle run and accessory loads. It has also been the practice to maintain the alternator charging voltage at a nominal value of 14.0 volts at an ambient temperature of 85 degrees F. The nominal value is raised to 14.6 volts at minus 20 deg. F. and lowered to 13.6 volts at 140 deg. F. This provides adequate charge current as a function of ambient temperature and thereby extends battery life.
A voltage regulator is used to inject a controlled current into the alternator rotor. This in turn provides a controlled current in the stationary (stator) field coils. This in turn yields the rectified dc output voltage required for battery recharge after start and to supply the required vehicle load currents.